1. Field of the Invention
The present invention relates to improvements in swab-type oil well pumping apparatus of the type dropped into a well for subsequent lifting, with a charge of oil, by natural gas pressure in the well.
2. Brief Discussion of the Prior Art
In many parts of the world, oil producing formations also produce natural gas in such large quantity that the gas interferes with the pumping of wells drilled into the formations by standard down-hole pumps operated by pump jacks at the earth's surface. Because of the evolution of large quantities of gas from such a formation, oil seeping into a well is whipped into a froth that will not close check valves in the down-hole pump. To solve this problem, inventors have developed pumps which can be dropped into a well to be subsequently lifted, with a charge of oil, by natural gas pressure in the well. In particular, one type of pump that has been developed is provided with a valve that is open as the pump falls into the well and is subsequently closed by hydrostatic pressure of liquids in the well. After the valve closes, a buildup of gas pressure in lower portions of the well drives the pump and oil above the pump to the surface for discharge of oil above the pump and relief of gas pressure below the pump so that the pump can again fall into the well for another pumping operation.
While pumps of this type provide a useful way of pumping gassy oil wells, difficulties have been encountered in their use in the past. A particular difficulty that has occurred in the past is that gas pressure below the pump has not been relieved when the pump reached the earth's surface with the result that the pump has hung up in the wellhead. Not only is the pump no longer operable in such cases, but attempts to reach the pump and eliminate the stoppage can be very dangerous because of the high pressure below the pump. For example, should the wellhead be opened, the pump and a large quantity of gas might be blown out of the top of the wellhead resulting in serious injury, or the death, of the well operator.
Another problem that has been encountered with prior art pumps of this type is related to sealing between the pump and the casing of a well in which the pump is used in order that natural gas pressure below the pump can lift the pump and a charge of oil. Several problems occur with respect to seals utilized in prior art pumping devices of this type. Initially, tubing from which an oil well casing is constructed is provided in a variety of weights with each weight of a standard size tubing having a wall thickness that is varied by varying the inside diameter of the tubing. Thus, a seal that fits one weight of casing tubing will not be well matched to another weight of casing tubing so that seals for the pump cannot be standardized. Accordingly, the sealing element that engages the interior wall of the casing must be selected on a well-by-well basis. Moreover, several different weights of casing tubing may be used in one well with the result that the sealing elements on the pump cannot be well matched to a particular well.
Additionally, the sealing elements that engage the casing of a well in pumps of this type are often subject to rapid deterioration arising from several sources. One source is the initial rate of fall of the pump through a well; that is, the rate of fall of the pump at a time when the pump is above the level of liquid in the well. For the fall to occur, well fluids must pass through a bore formed through the pump body and if the bore through the pump is set to permit easy passage of the pump through liquid when it reaches lower portions of the casing, the pump can reach very high speeds while falling through gas above the liquid level in the well. This high speed fall can result in excessive wear of the sealing elements by rubbing of the elements against the inner wall of the casing. On the other hand, the provision of some means to slow the passage of the pump through gas can cause the pump to settle very slowly through liquid resulting in a loss of efficiency in the pumping operation.
Finally, a problem with seals also arises from the assembly of a casing from lengths of tubing that are connected together end-to-end by means of couplings between the ends of the individual lengths of tubing. This manner of forming the casing results in gaps between the ends of adjacent lengths of tubing which, in the past, have not only given rise to accelerated wear of the seals but, in some cases, have inverted cup-shaped elements in the seals resulting in rapid deterioration of the sealing elements.